Risk factors for cancer from agricultural exposures in the ambient environment are studied to estimate the contribution of these environmental exposures to cancer in the rural population. Rural populations have exposures to pesticides and nitrates in drinking water that are intermediate between occupationally-exposed groups and the general population. Case-control studies have been conducted on non-Hodgkin's lymphoma and cancers of the brain, bladder, colon, rectum, stomach, esophagus, pancreas, and kidney. Databases of water contaminants, gathered for routine monitoring purposes, are being used to estimate past exposures via public water supplies to individuals in these case-control studies. Elevated nitrate levels in public drinking water supplies were associated with an increased risk of non-Hodgkin's lymphoma in Nebraska. An analysis of nitrate in drinking water in Minnesota where exposure levels were lower found no association. This hypothesis is being investigated further in ongoing case-control investigations of non-Hodgkin's lymphoma in Iowa, Seattle, and the Detroit area. The issue of nitrate as a cancer risk factor is being pursued further in an ecologic study encompassing twelve states in the United States and in a census-block group analysis in the Platte River Valley of Nebraska and Colorado, where elevated nitrate levels are among the highest in the country. We are also developing new approaches, and improving existing methods, of exposure assessment in studies of general environmental exposures, with a focus on pesticides and nitrate. These are required to better estimate risk and to detect the relatively small increases in risk often encountered in such studies. Geographic information systems (GIS) are being utilized for developing new approaches to estimating indirect exposure to pesticides and for estimating exposure to nitrates and pesticides in private wells. Remote sensing data (satellite imagery) and historical records were used to estimate the population in Nebraska and Iowa potentially exposed to agricultural pesticides through the proximity of their homes to agricultural land treated with pesticides. This approach is also being used to improve pesticide exposure assessment in California in conjunction with the State pesticide use reporting database. Validation studies are underway in Iowa and California to determine the relationship between pesticide levels in the home and the proximity to agricultural land where pesticides have been applied. Nitrate levels in private wells are being estimated using data on land cover, livestock feedlot locations, soil type, and other information.